The present invention relates to a cross-linkable, foamable polyolefin resin composition, and more particularly to a cross-linkable, foamable polyolefin resin composition showing excellent sheet moldability, mechanical properties, formability and heat resistance without substantially deteriorating the mechanical properties and heat resistance peculiar to polypropylene.
Polyolefin foams are excellent in mechanical strength, softness, hand, heat resistance, chemical resistance, etc., so that they are widely used for interior materials of automobiles, heat insulators, sporting goods, cushions for food wrappings, sound dampers, exterior materials, etc.
Among them, polyethylene foams have excellent softness, hand, elongation, etc. because of their low melting points. On the other hand, polypropylene foams are excellent in mechanical strength such as a tensile strength, a bending strength, etc. and a heat resistance, but because of their high melting points, they should be heated to a high temperature in the process of foaming or cross-linking, leading to the molecular scission of polypropylene, whereby large and uneven cells are likely to be formed. Moreover, since usual foaming agents tend to decompose at about 170.degree. C., they are not suitably used for polypropylene resins having high melting points. Also, since foaming agents having high decomposition temperatures can produce only small amounts of gas, a large amount of foaming agents are needed to obtain sufficiently high foaming ratios.
To obviate the problems inherent in the polypropylene resins, a proposal was made to provide a cross-linkable, foamable resin composition comprising a propylene-ethylene random copolymer and a linear low-density polyethlene (Japanese Patent Laid-Open No. 57-212236).
However, since a propylene-ethylene random copolymer having a large ethylene content is used in the above composition, it is difficult to obtain foams having sufficient high-temperature properties, particularly a heat resistance sufficiently high to carry out the forming process such as vacuum molding, stamping, etc.
Further, proposals were made to provide a composition comprising polyethlene and/or an ethylene copolymer such as a liner low-density polyethylene, etc., two types of polypropylene and/or propylene copolymers having melting points differing from each other by more than 10.degree. C., and a foaming agent; and a composition comprising a polypropylene resin containing 10-50 weight % of ethylene and the remaining amount of propylene, which has three or more melting peaks between 100.degree. C. and 165.degree. C., a polyethylene resin and a foaming agent (Japanese Patent Laid-Open Nos. 59-75929 and 64-87641.
However, in each of the above compositions, foams showing a good balance of a sheet moldability by low-temperature extrusion and high-temperature properties, particulary a heat resistance sufficient for carrying out vacuum molding, stamping, etc. can hardly be obtained.